1. Field of the Disclosure
Embodiments disclosed herein relate generally to process for upgrading petroleum feedstocks. In one aspect, embodiments disclosed herein relate to a process for hydrocracking and deasphalting resid. In another aspect, embodiments disclosed herein relate to an integrated process for upgrading resid including multiple hydrocracking stages.
2. Background
Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are useful, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil. Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique.
Crude oils range widely in their composition and physical and chemical properties. Heavy crudes are characterized by a relatively high viscosity, low API gravity, and high percentage of high boiling components (i.e., having a normal boiling point of greater than 510° C. (950° F.)).
Refined petroleum products generally have higher average hydrogen to carbon ratios on a molecular basis. Therefore, the upgrading of a petroleum refinery hydrocarbon fraction is generally classified into one of two categories: hydrogen addition and carbon rejection. Hydrogen addition is performed by processes such as hydrocracking and hydrotreating. Carbon rejection processes typically produce a stream of rejected high carbon material which may be a liquid or a solid; e.g., coke deposits.
Hydrocracking processes can be used to upgrade higher boiling materials, such as resid, typically present in heavy crude oil by converting them into more valuable lower boiling materials. For example, at least a portion of the resid feed to a hydrocracking reactor may be converted to a hydrocracking reaction product. The unreacted resid may be recovered from the hydrocracking process and either removed or recycled back to the hydrocracking reactor in order to increase the overall resid conversion.
The resid conversion in a hydrocracking reactor can depend on a variety of factors, including feedstock composition; the type of reactor used; the reaction severity, including temperature and pressure conditions; reactor space velocity; and catalyst type and performance. In particular, the reaction severity may be used to increase the conversion. However, as the reaction severity increases, side reactions may occur inside the hydrocracking reactor to produce various byproducts in the form of coke precursors, sediments, other deposits as well as byproducts which form a secondary liquid phase. Excessive formation of such sediments can hinder subsequent processing and can deactivate the hydrocracking catalyst by poisoning, coking, or fouling. Deactivation of the hydrocracking catalyst can not only significantly reduce the resid conversion, but can also require more frequent change-outs of expensive catalyst. Formation of a secondary liquid phase not only deactivates the hydrocracking catalyst, but also limits the maximum conversion, thereby resulting in a higher catalyst consumption which can defluidize the catalyst. This leads to formation of “hot zones” within the catalyst bed, exacerbating the formation of coke, which further deactivates the hydrocracking catalyst.
Sediment formation inside the hydrocracking reactor is also a strong function of the feedstock quality. For example, asphaltenes that may be present in the resid feed to the hydrocracking reactor system are especially prone to forming sediments when subjected to severe operating conditions. Thus, separation of the asphaltenes from the resid in order to increase the conversion may be desirable.
One type of processes that may be used to remove such asphaltenes from the heavy hydrocarbon residue feed is solvent deasphalting. For example, solvent deasphalting typically involves physically separating the lighter hydrocarbons and the heavier hydrocarbons including asphaltenes based on their relative affinities for the solvent. A light solvent such as a C3 to C7 hydrocarbon can be used to dissolve or suspend the lighter hydrocarbons, commonly referred to as deasphalted oil, allowing the asphaltenes to be precipitated. The two phases are then separated and the solvent is recovered. Additional information on solvent deasphalting conditions, solvents and operations may be obtained from U.S. Pat. Nos. 4,239,616; 4,440,633; 4,354,922; 4,354,928; and 4,536,283.
Several methods for integrating solvent deasphalting with hydrocracking in order to remove asphaltenes from resid are available. One such process is disclosed in U.S. Pat. Nos. 7,214,308 and 7,279,090. These patents disclose contacting the residue feed in a solvent deasphalting system to separate the asphaltenes from deasphalted oil. The deasphalted oil and the asphaltenes are then each reacted in separate hydrocracking reactor systems.
Moderate overall resid conversions (about 65% to 70% as described in U.S. Pat. No. 7,214,308) may be achieved using such processes, as both the deasphalted oil and the asphaltenes are separately hydrocracked. However, the hydrocracking of asphaltenes as disclosed is at high severity/high conversion, and may present special challenges, as discussed above. For example, operating the asphaltenes hydrocracker at high severity in order to increase the conversion may also cause a high rate of sediment formation, and a high rate of catalyst replacement. In contrast, operating the asphaltenes hydrocracker at low severity will suppress sediment formation, but the per-pass conversion of asphaltenes will be low. In order to achieve a higher overall resid conversion, such processes typically require a high recycle rate of the unreacted resid back to one or more of the hydrocracking reactors. Such high-volume recycle can significantly increase the size of the hydrocracking reactor and/or the upstream solvent deasphalting system.
Accordingly, there exists a need for improved resid hydrocracking processes that achieve a high resid conversion, reduce the overall equipment size of hydrocracking reactor and/or solvent deasphalter, and require less frequent hydrocracking catalyst change-outs.